1. Field of the Invention
The present invention relates generally to methods for reducing fruit softening. In particular, it relates to methods for reducing fruit softening and cell wall polysaccharide degradation by inhibiting the activity of one or more endo-1,4-.beta.-glucanase
2. Information Disclosure
Ripening, the final phase of fruit development, involves a number of dramatic metabolic changes in fruit tissue. An important aspect of the ripening process is fruit softening, which is thought to result primarily from modifications of the cell wall. Many subtle changes in metabolic activity are involved in this response.
The prior art discloses ripening-impaired mutants, such as the rin mutant which have been used to study fruit ripening. Tigchelaar Hortic. Sci., 13:508-513, 1978. The use of these mutants to specifically control fruit softening has met with limited success, however, because of the pleiotropic nature of these mutations.
An increase in the activity of polygalacturonase, an enzyme responsible for the degradation of pectin, has been correlated with fruit softening. Recombinant constructs have been prepared containing a plant promoter linked to polygalacturonase cDNA in the antisense direction. These constructs have been inserted into tomato to inhibit the activity of this enzyme in ripening fruit. Smith et al., Nature, 334:724-726, 1988; Sheehy et al., Proc. Nat. Acad. Sci., 85:8805-8809, 1988; Hiatt et al., U.S. Pat. No. 4,801,340; Bridges et al., EPO Publication No. 0,271,988. Although these constructs have been shown to inhibit polygalacturonase activity, an effect on fruit softening has not been shown. Smith et al.,Plant Mol. 14:369-379, 1990.
Endo-1,4-.beta.-glucanase is another enzyme thought to be involved in fruit softening. It is known to degrade the major hemicellulosic polymer, xyloglucan. Hatfield and Nevins, Plant and Cell Physiol., 27:541-552, 1986. The cDNA and gene encoding endo-1,4-.beta.-glucanase have been cloned from avocado (Christoffersen et al., Plant Molec. Biol., 3:385, 1984) and bean (Tucker et al., Plant Physiol., 88:1257, 1988), both of which are incorporated herein by reference.